White clouded vitreous enamel



Patented May 24, 1938 earner OFFlCE WHITE CLOUDED VITREOUS ENAMEL Ignaz Kreidl, Vienna, Austria No Drawing. Application August 21, 1936, Serial No. 97,288. In Austria February 25, 1936 9 Claims.

This invention relates to the production of white clouded iron enamel, more particularly enamel clouded white by means of gas clouding. Proposals have already been made to influence the clouding by changing the composition of the enamel charge. Thus it has been proposed by changing the composition of the enamel charge to raise the colloidal properties of the frit itself in the slip during normal grinding, e. g. by raising 1) the content of fluorine compounds, with exception of cryolite, or by changing the proportion of alkali to boric acid in such a manner that this proportion is greater than in the usual enamel compositions containing, with normal fluorine content, alkali to boric acid in the proportion of approximately 1 parts of alkali to 1 part of boric acid. A ratio of above 2:1 up to about 5:1 has been found to be suitable, in the reckoning of which ratio however the alkali content of any alkali-containing fluorine compound which may have been added is not included.

The present invention depends on the discovery that with cryolite enamel also effects can be produced which are advantageous for gas cloudings when on the one hand the fluorine content introduced into the enamel by means of cryolite is not less than 8% (preferably is between 8 to 12% fluorine), and on the other hand the enamel at approximately normal boric acid content is as free as possible from aluminium oxide, which is not introduced by cryolite, whereas at low boric acid content, more particularly in the case of an abnormally low such content, an addition of aluminium oxide may be permitted if such an addition is desirable for any reasons.

The subject of this invention accordingly is an enamel in which the fluorine has been introduced in the usual manner by means of cryolite to an extent such that the batch does not contain less than 8% fluorine, and preferably contains between 8 and 12% thereof, the said batch however, with a boric acid content approximately normal or not deviating far from normal, more particularly with a boric acid content having a maximum of 10%, preferably however with a boric acid content less than 10%, being as free from aluminium oxide as possible, not reckoning in the aluminium oxide which has been introduced by cryolite, whilst on the other hand if aluminium oxide must be introduced into the enamel for any reason a part of the cryolite is replacedby a fluorine compound which is free from aluminium oxide, such as more particularly alkali fluoride, and preferably alkali silicofluoride, or the boric acid content must be correspondingly lowered even below the normal boric acid content, and in fact must be lowered the more the greater the amount of aluminium oxide introduced into the enamel, both expedients being combined with one another if desired. 10

Thus for example with a boric acid content of approximately 6% aluminium oxide may be introduced into the enamel up to a maximum of 7% without the enamel becoming unsuitable for use, the amount introduced being smaller "the more the boric acid content exceeds 6%, and 'at a 5 boric acid content of approximately 10% the aluminium oxide content, not reckoning in the aluminium oxide introduced by means of cryolite, is as far as possible zero. If however the portion of boric acid or aluminium oxide, either alone or 0 both together, exceeds an upper limit, then it is not possible to produce sufficiently strong gas cloudings which show the insensitivity towards overburning which is necessary for practice.

In carrying out the process the fusion of the raw mixture, or the fritting, is to be carried out so that the burning off of fluorine is as low as possible. In the case of an enamel frit according to the invention the fluorine addition consequently also depends on the way the fusion process is carried out, and in fact the fluorine content of the enamel charge must be higher the higher the fusion temperature is at which the enamel charge is fritted or fused together, or the longer the duration of the fusion process, because all fluorides on fusion in the enamel charge exhibit the so-called fluorine burning-off phenomenon, which is a loss of fluorine.

It has been found advisable for the content of soda or the like not to exceed 8%. 4 It has further been found that the earth oxides and the alkaline earth oxides or compounds thereof, such as for example the oxides and compounds of calcium, magnesium and so forth be- 45 have similarly to those of aluminium, but in general they are already injurious in far smaller quantities.

The enamel frits according to the invention are suitable more particularly for coating enamel, 50

that is to say for enamels which are free from adhesive oxides like oxides of cobalt and nickel, and in fact are suitable for the production of white clouded iron. enamel, more particularly such enamel which is clouded White by gas clouding.

The following three examples may be given of enamel mixtures which are fritted together in accordance with the invention:-

1. Borax 22 Cryolite 18 Soda 4 Saltpetre 2 Quartz 54 Felspar 2. Borax 14 Cryolite 16 Soda 4 Saltpetre 3 Quartz "'43 Felspar 20 3. 'Borax 22 Cryolite 12 Sodium 'silicofluoride 4 Soda .4

Saltpetre- 3 Quartzn 40 Felspar 15 In these examples the fusion process should not be conducted too long nor at too high a temperature so as to avoid too' great burning off of fluorine. c

Iclaim::. a

1. A vitreous enamel frit for the production of white clouded vitreous enamels for ironware, resulting from the melting and grinding of a mixture of ingredientscontaining at least silicates, borates, fluorine compounds and aluminium compounds,'said mixture containing cryolite in a proportion to provide elemental fluorine in a quantity of substantially 8% to 12% of the mixture, boron compounds in a proportion to provide boric acid in an amount not exceeding 10% of the mixture, and an aluminium oxide content derived from sources other than cryolite which is not greater than 7%, the amount of aluminium oxide from sources other than cryolite varying substantially from zero upward as the amount of boric acid decreases from 10% downward.

2. A vitreous enamel frit according to claim 1 which is substantially free from aluminium oxide derived from sources other than cryolite.

3. A vitreous enamel frit according to claim 1 containing less than 8% soda.

4. A vitreous enamel frit according to claim 1 which is free from compounds of substances selected from the group consisting of earth oxides and alkaline earth oxides.

5. A vitreous enamel frit for the production of white clouded vitreous enamels for ironware, resulting from the melting and grinding of a mixture of ingredients containing at least silicates, borates, fluorine compounds and aluminium compounds, said mixture containing cryolite in a proportion to provide elemental fluorine in a quantity of substantially 8% to 12% of the mixture, 7 boron compounds in a proportion to provide boric acid in an amount not exceeding 10% ofthe mixture, and an aluminium oxide content derived from sources other than cryolite which is not greater than 7%, the amount of aluminium oxide from other sources than cryolite varying substantially from zero upward as the amount of boric,

acid decreases from 10% to 6%.

6. A vitreous enamel frit for the production of white clouded vitreous enamels for ironware, resulting from the melting and grinding ofa mixture of ingredients containing at least silicates,

borates, fluorine compounds and aluminium coming in direct proportion to the amount of aluminium 'oxide present in aluminium compounds other than cryolite, said mixture further containing boron compounds in a proportion to provide boric acid in an amount not exceeding 10% V of the mixture.

'7. A vitreous enamel frit according to claim 1 in which the melting of the crude enamel mixture has been so conducted that burning off of fluorine is kept low;

containing a gas clouding agent and a vitreous enamel frit as claimed in claim 1.

IGNAZ KREIDL. 

